Processes for preparing polyol fatty acid polyesters, including processes that utilize solvent-free transesterification reactions, have been described in U.S. Pat. No. 3,963,699, Rizzi et al., issued Jun. 15, 1976; U.S. Pat. No. 4,517,360, Volpenhein, issued May 14, 1985; and U.S. Pat. No. 4,518,772, Volpenhein, issued May 21, 1985. Additional patents describing processes for preparing lower and higher esters of polyols include U.S. Pat. No. 2,893,990, Hass et al., issued Jul. 7, 1959; U.S. Pat. No. 3,251,827, Schnell et al., issued May 17, 1966, which discloses that the particle size of the sugar should be kept small to avoid formation of higher esters; U.S. Pat. No. 3,558,597, Brachel et al., issued Jan. 26, 1971; U.S. Pat. No. 3,644,333, Osipow et al., issued Feb. 22, 1972; U.S. Pat. No. 3,792,041, Yamagishi et al., issued Feb. 12, 1974, all which disclose making a solution of sucrose and fatty acid soap in water and adding the fatty acid ester and catalyst before elevating the temperature to drive off the water; U.S. Pat. No. 4,032,702, James, issued Jun. 28, 1977, which discloses using lower esters of sucrose as emulsifiers in the preparation of lower esters and the use of soap as a catalyst for such reactions; U.S. Pat. No. 4,298,730, Galleymore et al., issued Nov. 3, 1981, which also discloses the use of soap as an emulsifier and catalyst; U.S. Pat. No. 4,334,061, Bossier et al., issued Jun. 8, 1982, which discloses the use of a water washing step to purify the polyol polyester and incidentally discloses the use of inert gas sparging to remove lower alcohol from the reaction between sucrose and lower alkyl ester of fatty acid to speed the reaction and the removal of unreacted sucrose from an initial stage of a batch reaction for no indicated reason; and U.S. Pat. No. 4,877,871, Klemann et al., issued Oct. 31, 1989. All of said above patents are incorporated herein by reference.
Many of the above patents teach processes that use solvents to assist in the formation of a homogeneous reaction mixture. However, the solvent must ultimately be removed before the polyester can be ingested. Also, many of the processes involving solvents are primarily for preparation of esters having only a low degree of esterification, i.e. containing one or two ester groups, that are useful as surfactants.
Using alkaline materials in the process for making polyol fatty acid polyesters is known in the art. For example, U.S. Pat. No. 3,198,784, Griscom et al., issued Aug. 3, 1965, discloses adding sodium hydroxide to an aqueous solutions of sucrose as a preliminary step for making benzylated sucrose. U.S. Pat. No. 3,849,3412, Lamberti, issued Nov. 19, 1974 discloses using alkaline solutions for making ester-linked derivatives of carbohydrates. European Patent Applications 319,091 and 319,092, both assigned to Unilever and published Jun. 7, 1989, disclose methods for making light color, polyol fatty acid oils by utilizing alkaline solutions in the purification steps.
European patent application 319,092 discloses rinsing the crude polyol polyesters with an alkaline solution, having a pH of at least 12.5, at temperatures from 40.degree. C.-110.degree. C. European patent application 319,091 discloses a process for lowering the level of residual alkali metal ions left over from the base catalyzed synthesis and/or the purification proceedure disclosed in European patent application 319,092. The process is aimed at reducing the level of alkali metals in the polyol fatty acid ester to no more than 5 ppm by weight of the finished product. Although the light colored, polyol fatty acid polyester oils made therein are reported to have improved keepability, no improved oxidative stability of said oils is disclosed.
The present invention relates to processes for making polyol fatty acid polyesters having improved oxidative stability. These polyol fatty acid polyesters may be used, for example, to make cooking oils having superior flavor and odor, longer shelf-life, and longer fryer-life.